ci: sign release artifacts, container images, and SBOMs with cosign

[ibourgeois]

Context

As a maintainer, I need CI to sign Katra release artifacts, published container images, and generated SBOMs with cosign so downstream users can verify artifact integrity and provenance.

Acceptance Criteria

• CI signs published container images with cosign.
• CI signs release artifacts that are distributed to end users where cosign fits the release flow.
• CI signs generated SBOMs or otherwise produces verifiable attestations for them.
• The verification path is documented well enough for downstream users and maintainers to validate signatures.
• The workflow fails clearly when signing or attestation steps break.

Notes

This should build on GitHub-hosted identity or another supported key management approach that works cleanly with the existing release automation.

Out of Scope

• Replacing existing Apple signing and notarization requirements for macOS apps
• Building a separate trust portal or verification service